This invention relates to personal computers capable of having the usual system controlling processor reset, initialized and then isolated if an alternate system controller is provided for the system and, more particularly, to such a computer in which such transfer of control to an alternate system controller may be overridden in the event of failure of the alternate system controller.
Personal computer systems in general and IBM personal computers in particular have attained widespread use for providing computer power to many segments of today's modern society. Personal computer systems can usually be defined as a desk top, floor standing, or portable microcomputer that consists of a system unit having a single system processor and associated volatile and non-volatile memory, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage device, and an optional printer. One of the distinguishing characteristics of these systems is the use of a motherboard or system planar to connect these components together. These systems are designed primarily to give independent computing power to a single user and are inexpensively priced for purchase by individuals or small businesses. Examples of such personal computer systems are IBM's PERSONAL COMPUTER AT and IBM's PERSONAL SYSTEM/2 Models 25, 30, 35, 40, L40SX, 50, 55, 57, 65, 70, 80, 90 and 95.
These systems can be classified into two general families. The first family, usually referred to as Family I Models, use a bus architecture exemplified by the IBM PERSONAL COMPUTER AT and other "IBM compatible" machines. The second family, referred to as Family II Models, use IBM's MICRO CHANNEL bus architecture exemplified by IBM's PERSONAL SYSTEM/2 Models 50 through 95. In the beginning, the Family I models typically used the popular INTEL 8088 or 8086 microprocessor as the system processor. These processors have the ability to address one megabyte of memory. Later Family I models and the Family II models typically use the higher speed INTEL 80286, 80386 and 80486 microprocessors which can operate in a real mode to emulate the slower speed INTEL 8086 microprocessor or a protected mode which extends the addressing range from 1 megabyte to 4 Gigabytes for some models. In essence, the real mode feature of the 80286, 80386, and 80486 processors provide hardware compatibility with software written for the 8086 and 8088 microprocessors.
As personal computer technology has developed and moved from eight to sixteen and eventually thirty two bit wide bus interaction and higher speed microprocessors capable of real and protected mode operation, performance capability has been sought by separating the architecture of the personal computer into varying bus areas. More specifically, in the original IBM PC, what came to be known as the expansion bus was essentially a direct extension of the microprocessor (8086 or 8088) connections, buffered and demultiplexed as required. Later, as the AT bus specification was developed and came into wide use (now being also known as the Industry Standard Architecture or ISA), it became possible to sever the nearly direct connection between the microprocessor and the bus, giving rise to the presence of what became known as the local processor bus and the renaming of the expansion bus as the input/output bus. Typically, in order to enhance performance, the local processor bus runs at a higher clock speed (typically expressed in Hertz) than does the input/output bus. The IBM AT architecture also opened the possibility of running more than one microprocessor on the input/output bus, through use of direct memory access (DMA) interrupts.
As enhanced performance capability has continued to be a goal, and as faster clock speeds have become attainable for microprocessors, strategies have evolved in which it has become desirable to accommodate alternate system controllers which may, under appropriate circumstances, assume control of systems into which the controllers are inserted. For example, the provision of a processor upgrade card or board is one example of such a strategy. Heretofore, such strategies have usually contemplated the substitution of the upgraded component for the component previously used. Such substitution is acceptable where components are received in socket connectors permitting ready interchange. However, where components are soldered in place on circuit boards (as is the case with a surface mount Intel 80386SX) such substitution is less feasible or impossible. In particular, mere addition of another device normally will result in unacceptable contention for control of system resources and bus access.